1. Field of the Invention
The present invention relates to a process for producing solar cells and solar cells produced thereby, and more particularly relates to a process for producing solar cells for use in outer space which can be borne by an artificial satellite and solar cells produced by the process.
2. Description of Related Art
In recent years, solar cells which can be used in outer space are under active development.
Solar cells in use in space are exposed to cosmic rays flying around in huge numbers. As a result, the solar cells deteriorate in properties including conversion efficiency with varying degrees of deterioration depending upon the kind of the solar cells. Accordingly, it is one of the most important challenges in development of space-use solar cells to realize the highest possible output for an final output (i.e., minimum output during their service) in view of irradiation doses in a use environment.
Typical space-use solar cells are produced by the following process:
First, as shown in FIG. 12(a), a semiconductor substrate 50 is used. The semiconductor substrate 50 is cut from an ingot of single-crystal silicon usually in the form of a wafer of about 300 .mu.m thickness.
Next, as shown in FIG. 12(b), this substrate 50 is made into a thin-film silicon substrate 51 of about 100 to 200 .mu.m thickness by etching with an aqueous acidic or alkali solution or by polishing. Subsequently, the thin-film silicon substrate 51 is washed a number of times. Then, p-type diffusion layers (not shown) are formed on one surface of the silicon substrate 51, for example, on a surface opposite to a photo-receptive face-to-be (i.e., on a non-photo-receptive face).
Subsequently, high-concentration n-type diffusion layers (not shown) are formed on the photo-receptive face. Thereafter, as shown in FIG. 12(c), an insulating film is partially removed, and n-electrodes 52 of Ag/Pd/Ti and p-electrodes (not shown) of Ag/Pd/Ti/Al are formed on the photo-receptive face and on the non-photo-receptive face, respectively.
Then, as shown in FIG. 12(d), the resulting substrate is separated into individual cells. Thus solar cells of single-crystal silicon are obtained.
Also solar cells in which both electrodes contact on a rear surface are well known. The solar cells of this type are produced by the following process:
First, a semiconductor substrate 50 as shown in FIG. 13(a) which is cut in the form of a wafer as in the above-described process is made into a thin-film silicon substrate 51 as illustrated in FIG. 13(b). Subsequently, the silicon substrate 51 is subjected to a number of washings.
Thereafter, on one surface of the silicon substrate 51, for example, on the non-photo-receptive face thereof, p-type diffusion layers and n-type diffusion layers are formed in the form of islands.
Then, as shown in FIG. 13(c), an insulating film is partially removed, and electrodes, for example, of Ag/Pd/Ti/Al are formed as p-electrodes 54 and n-electrodes 52 so as to come in contact with the island-like p-type and n-type diffusion layers, respectively.
Subsequently, as in the aforementioned process, the resulting substrate is divided into individual cells, as shown in FIG. 13(d). Thus solar cells of single-crystal silicon are obtained.
Generally, the thinner the substrate of a solar cell is, the less susceptible to the effect of cosmic rays the solar cell is.
However, in the case of a substrate of a crystalline semiconductor, the thinner the substrate is, the stronger the possibility of the substrate breaking during manufacture of solar cells becomes. Therefore, it is actually impossible to manufacture thin solar cells from a very thin substrate of a crystalline semiconductor. In a conventional technique, the marginal thickness of the substrate is about 100 .mu.m. Furthermore, in this case, it is required to process an originally thick substrate into a substrate of about 100 .mu.m thickness before entering the process of constructing a solar cell structure.
On the other hand, in the case where an amorphous semiconductor is used, it is possible to obtain very thin solar cells. This is because the amorphous semiconductor can usually be formed in extreme thin films on transparent substrates by CVD and the thin films can be used for the solar cells. In addition to that, the amorphous semiconductor can have relatively free band gaps, and accordingly is excellent in radiation resistance. However, the films of the amorphous semiconductor include lots of elements nucleating re-association of minority carriers such as in-gap levels and grain boundaries within the full area of the films, and the films of the amorphous semiconductor have a shorter diffusion length for carriers, which greatly affects the conversion efficiency of solar cells, compared with single-crystal silicon. Therefore, a high initial conversion efficiency cannot be expected unless a breakthrough technique is introduced to the thin films.
In contrast, with solar cells of crystalline materials typified by silicon, a high initial conversion efficiency before exposure to radiation can be realized. However, every crystalline material has its own fixed band gap. In addition to that, a thin substrate thereof is difficult to handle as described above. Therefore, it has been very hard to improve the radiation resistance of the solar cells of this type.